I. Field of the Invention
The present invention relates to methods and apparatus for establishing electrical connection to at least one conductor which has an exposed surface defining in part an at least partially enclosed space. Electrical connection may also be established to at least one electrical device located outside the space. Preferably the subject methods and articles are applicable to establishing electrical contact in multilayer circuit boards.
II. Description of the Prior Art
The present day electronic device industry is based in large part on the use of printed circuits boards. In the simplest form these boards essentially comprise one sheet of plastic or other insulating base layer with layers of electrical conductors adhered to one or both sides of the sheet of plastic. The conductive layers establish various electrical circuit patterns. Holes are drilled through the sheet of plastic and conductive layers for mounting electrical devices to the broad. More sophisticated circuit boards may consist of superimposed layers of alternating plastic sheets and conductive layers laminated one on top of another to form a single structure. The conductive circuitry found in the various layers of such advanced circuit boards must be selectively interconnected in order to complete the desired circuit design. Furthermore, various conductive layers including internal conductive layers located between the laminated sheets of plastic must be capable of being selectively connected to external electrical components.
For simple printed circuit boards having adhering circuit conductors on both exposed surfaces and having a hole drilled through the board exposing a pathway between the two conductors, an interconnection between the conductors can be achieved by the employment of pressed-in conductive grommets and/or hollow rivets. The grommets or rivets are inserted into the hole and bent over at each end to provide a rigid mechanical and electrical interconnection between the circuit conductors on the exposed surfaces of the board. However, the use of pressed-in grommets and/or hollow rivets is obviously confined to double side single layer circuit boards since assured connection to internally located conductors is at best uncertain.
Perhaps the most widely commercially acceptable method of providing electrical connection to conductive circuitry in advanced circuit boards consists of forming an electroplated coating on the interior walls of the holes drilled through such advanced boards to expose surfaces of selected conductors. These coatings serve two purposes. The first purpose is to electrically interconnect conductors at various levels within the circuit boards to each other and to conductors located on the surface of the circuit boards. The second purpose is to facilitate the connection of electrical devices to selected circuit conductors on and within the circuits boards by allowing for the insertion of wire leads from these electrical devices into the electro-plated holes. Thereafter, the board is passed over an up-welling wave of molten solder and the solder is flowed up through the holes to provide surface connection between the electro-plated coating and the inserted wire leads. The metal coating of the hole wall serves as a thermally conductive pipe supplying heat drawn from the wave of molten solder to the liquid solder advancing through the hole.
While providing a mechanism for achieving fairly consistent electrical connections to internally located circuit conductors, the process of plating copper onto hole walls formed within a circuit board involves the performance of a number of mechanical, chemical, electro-chemical, photo-chemical, and sometimes plastic-lamination steps. For example, a tape-controlled machine will typically perform the drilling of the holes and thereafter panels on which the circuit board are formed are degreased, deburred by sanding, and the holes are honed with an abrasive blast before any chemical processing. The circuit board panels then enter a first chemical process line which operates to create a thin seed coating of conductive copper on the internal walls of the holes and on external copper surfaces of the circuit board. This seed coating can be between 20 to 100 or more micro inches thick depending upon the nature of the chemical baths. The seed coating is a base upon which a thicker conductive layer of copper can be electro-plated over the exposed plastic surfaces in the hole. The sequence of steps to achieve the seed coating is as follows:
(1) the panels are manually racked in the seed coating process line; PA1 (2) a cleaner is applied to the panels through a dip process; PA1 (3) the panels are rinsed; PA1 (4) a conditioner is applied through a dip process; PA1 (5) the panels are rinsed; PA1 (6) the panels are surfaced etched through a dip process; PA1 (7) the panels are rinsed; PA1 (8) the panels are exposed to sulfuric acid in a dip process; PA1 (9) the panels are rinsed; PA1 (10) the panels are exposed to a hydrochloric acid dip; PA1 (11) the panels are exposed to a catalyst solution dip; PA1 (12) the panels are rinsed; PA1 (13) the panels are re-rinsed; PA1 (14) the panels are re-racked manually; PA1 (15) the panels are acid dipped; PA1 (16) the panels are rinsed; PA1 (17) the panels are exposed to an alkaline copper solution dipping; PA1 (18) the panels are rinsed; PA1 (19) the panels are re-rinsed; and PA1 (20) the panels enter an acid holding tank. PA1 (21) the panels are transferred manually to a plating rack; PA1 (22) the panels are dipped into an acid bath; PA1 (23) the panels are rinsed; PA1 (24) plating is commenced at a low current density; PA1 (25) plating is continued at an increased current density; PA1 (26) the panels are rinsed; PA1 (27) the panels are dried; PA1 (28) the panels are unracked manually.
At this point the entire panels, all surfaces, edges, and hole walls have been coated with metallic copper in thickness sufficient to be an effective conductive seed coating to plate on a final layer of copper. The sequence of steps to achieve the copper plating is as follows:
Of the 28 steps identified above, four involve manual operations and the remainder could be manual or automated under tape or computer control. These 28 steps result in a coating of copper of 1 to perhaps 2 mils thick. The coating serves to establish contact from outer conductive surface to outer conductive surface or from outer conductive surface to an internal exposed conductor and acts as a thermal conductor during a subsequent wave soldering step. This brief description of a presently commercially available method of providing electrical contact to an internal circuit conductor of a printed circuit board does not include consideration of the financial, solution control, rinse water supply control, waste solution disposal control, or other logistic considerations needed to maintain a high yield production schedule.
Although not known to be commercially viable, alternative methods of providing electrical connection to internally located conductors in a printed circuit board have been disclosed in issued U.S. patents. For example, in U.S. Pat. No. 3,040,119 issued to Granzow, an electrical circuit board is disclosed in which powdered or grandular grains or particles of an electro-conductive metal are compressed under pressure into each hole of a circuit board. Preferably, particles of copper are disclosed as being employed in conjunction with any suitable mechanism which is capable of tightly compressing the metal particles into holes within the circuit board so that the particles will be solidified into a rigid mass and permanently anchored in the holes. It is also taught in Granzow that the compressed solidified particles may be established in the form of a sleeve or cylinder lining the hole of the circuit board and that the lead of an electrical device may be inserted in the middle of the sleeve or cylinder and solder run down into the sleeve or cylinder to thereby make a solder connection between the lead of the electric device and printed circuit conductors on the surface of the printed circuit board. A primary disadvantage of such a method resides in the reliance on a pure mechanical connection between the exposed conductors on the printed circuit board and the compressed particulate mass.
In U.S. Pat. No. 3,158,503 issued to Young a flowable metal paste in the form of a thick "cream" containing silver powder is wiped across the surface of a first board to fill a hole within the board. The hole of the first board is then aligned over a hole of a second board and jets of compressed air are passed over the first board to force the silver paste onto the inner surface of the hole of the second board. This and other methods of depositing a paste held in a fluid bind all suffer from the deficiency of reduced conductivity due to contact of the non-conductive binder against the exposed surface of the circuit board conductor.
U.S. Pat. No. 3,561,110 issued to Feulner et al. attempts to overcome the deficiency of such prior art methods of providing conductive paths through very small vertical and/or transverse holes in non-metallic bodies such as printed circuit boards by filling such holes with metallic particles ranging from approximately 0.5 to 3 mil in diameter and then sintering the dry metal particles. Feulner et al. further teaches that a molten metal such a solder may be applied to the sintered-produced conductive plug at the surface of the hole and allowed to permeate through the spaces between the sintered particles. This particular method has the disadvantage of having to subject the nonmetallic portions of the printed circuit board to a sufficiently high temperature for a sufficiently long amount of time to achieve the sintering of the metallic particles. In addition, subsequent application of a molten media such as solder to the surface of the hole would appear to result in only a limited flow throughout the sintered particles.
The teachings of U.S. Pat. No. 3,616,532 issued to Beck attempt to resolve the difficulties of providing an electrical connection to internally exposed conductors in a printed circuit board through the employment of a spring held in compressed state by a coating of solder. The compressed spring is inserted into an aperture in an insulating substrate which is positioned between layers of circuitry which are to be inter-connected. Upon heating, the solder is reported to melt to permit the spring to expand and thereby establish contact between the conducted surfaces. When the solder again cools, the connection is reportedly established. The obvious limitations of such a method of inter-circuit connection lie in the difficulty of accurately aligning such spring assemblies with small diameter circuit board interconnecting holes. Furthermore, an inventory of springs of various diameters and lengths is inevitably required.
U.S. Pat. No. 3,999,004 issued to Chirino et al. approaches the problem by positioning a metal paste within holes of a circuit board by mounting the circuit board on a rotatable wheel. The wheel is then spun to subject the paste to centrifugal forces which reportedly acts to remove excess paste and to uniformly and evenly coat the interior side walls of holes in a laminar stack. This approach not only suffers from the above-mentioned diffusing inherent in all "metal paste" methods but also requires that the board be subjected to substantial mechanical forces.
The above-mentioned attempts at providing electrical circuit connections to printed circuit boards are representative of attempts to devise an economically suitable solution to the problem which fall short of success.
It is, accordingly, an object of the present invention to provide methods and apparatus for achieving electrical connections to internal conductors of circuit boards which are highly reliable.
It is another object of the present invention, to provide methods and apparatus for producing suitable internal circuit board electrical connections with elimination of many of the steps and expenses inherently involved with copper-electro plating electrical connections.
It is still another object of the present invention to provide a means for simply and efficiently coupling electrical device to the internal conductors of a circuit board which does not subject the circuit board to high temperature, destructive chemicals and/or substantial mechanical forces.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.